FIG. 4 illustrates a prior art solenoid drive circuit which comprises a DC power source 1; a switching element or MOSFET 10 and a solenoid 4 connected in series to DC power source 1; a PWM (Pulse Width Modulation) controller 23 for supplying drive signals S23 of controlled pulse width to a control or gate terminal of n-channel MOSFET 10 to alternately turn on and off MOSFET 10; and a control circuit 22 for producing command signals S25 to PWM controller 23. Control circuit 22 has a detector 6 composed of a resistor 6a which detects current flow I1 through solenoid 4 to produce an electric voltage equivalent to the level of current flow I1. Positive and negative terminals of DC power source 1 are connected respectively to a main or drain terminal of MOSFET 10 and earth. For example, the other main or source terminal of MOSFET 10 is grounded through solenoid 4 and detector 6 of control circuit 22. PWM controller 23 produces drive signals S23 to gate of MOSFET 10 to alternately turn on and off MOSFET 10 under control of current flow I1 through solenoid 4.
Control circuit 22 also includes an amplifier 7 for amplify the electric voltage applied on resistor 6a; a retention circuit 32 for sampling and retaining outputs S7 from amplifier 7; an A/D converter 13 for converting a voltage value S32 in retention circuit 32 into digital values S13; and an operational comparator or CPU 25 for comparing digital values S13 from A/D converter 13 with an objective current flow value to produce to PWM controller 23 outputs S25 indicative of the objective current flow value for solenoid 4. Source terminal of MOSFET 10 is connected to a cathode terminal of a diode 11 and an end of solenoid 4, and an anode terminal of diode 11 is grounded. Resistor 6a has one end connected to solenoid 4 and the other end connected to ground to detect current flow through solenoid 4. Both ends of resistor 6a are also connected to inverted and non-inverted terminals of amplifier 7 whose output terminal is connected to an input terminal of retention circuit 32. Output terminal of retention circuit 32 is connected to an input terminal of A/D converter 13 whose output terminal is connected to an input terminal of operational comparator 25.
Operational comparator 25 receives output signals S22 from at least one of sensors not shown such as an accelerator sensor, speed sensor, hydraulic pressure sensor or other sensors equipped in an automobile to calculate an objective current flow value for solenoid 4 based on output signals S22. Also, operational comparator 25 counts digital signals S13 indicative of detected current flow values from A/D converter 13, calculates a mean current flow value, and produces instruction signals S25 representative of current flow indication based on the mean current flow value and objective current flow value so that operational comparator 25 supplies instruction signals S25 to an input terminal of PWM controller 23 to determine a duty ratio of drive signals S23 to MOSFET 10 in PWM controller 23 in view of instruction signals S25 of current flow indication. Specifically, operational comparator 25 serves to optimize the level of current flow through solenoid 4 such that operational comparator 25 decreases current flow indication of instruction signals S25 when the detected current flow value is higher than the current objective value, and therefore, PWM controller 23 receives the decreased current flow indication of instruction signals S25 and lowers or narrows the duty ratio of drive signals S23 to MOSFET 10 accordingly to reduce the current flow through solenoid 4. Adversely, when the detected current flow value is lower than the current objective value, PWM controller 23 boosts or widens the duty ratio of drive signals S23 to MOSFET 10 based on current flow indication of instruction signals S25 to increase the current flow through solenoid 4. Thus, operational comparator 25 produces current flow indication of instruction signals S25 based on voltage value applied on resistor 6a, and thereby, PWM controller 23 produces drive signals S23 of controlled pulse width to MOSFET 10 to optimize current flow through solenoid 4.
For example, Japanese Patent Disclosure No. 2002-319506 discloses a solenoid drive circuit with a retention circuit as shown in FIG. 4. Detected voltage held in retention circuit 32 is converted into digital values through A/D converter 13 and supplied to operational comparator 25 which then calculates mean value of current flow from the detected voltage of digital value to produce to PWM controller 23 instruction signals S25 of current flow indication based on a deviation between the calculated mean value and objective current flow value. Accordingly, responsiveness of the system is determined by point number or length of point interval of sampling.
It should be noted that the solenoid drive circuit shown in FIG. 4 is defective because it cannot be operated with accuracy when resistor 6a detects a noise-superimposed voltage associated with switching operation of MOSFET 10 or a surge or voltage movement due to sudden voltage fluctuation of power source. Although a large amount of data can be sampled with retention circuit 32 for a short period of time to calculate mean current values and improve control accuracy of solenoid 4, however, this method increases number of the computation due to processing of the large amount of data and results in increased operation load on operational comparator 25.
Another solenoid drive circuit shown in FIG. 5 comprises an integrator 38 in lieu of retention circuit 32 of FIG. 4 to avoid inaccuracy in operating the solenoid drive circuit because integrator 38 can absorb noise to some extent. However, the solenoid drive circuit indicates the degraded responsiveness or deteriorated controllability with overshoot or undershoot when current flow is varied in response to external factors in case integrator 38 involves a large time constant for electric charge and discharge.
An object of the present invention is to provide a solenoid drive circuit capable of controlling current flow through a solenoid in good responsiveness to detected values of current flow through the solenoid without increase of operation load to be processed in the solenoid drive circuit. Another object of the present invention is to provide a solenoid drive circuit capable of driving a solenoid with an improved controllability without processing a large amount of current flow data on the solenoid.